Many current electronic communication systems use quadrature modulation schemes, which involve in-phase (I) and quadrature (Q) signal components and do not have constant envelopes. Examples of such communication systems are cellular radio telephone systems that use wideband code division multiple access (WCDMA), orthogonal frequency division multiple access (OFDMA), and their variants. Thus, part of the communicated information is encoded in the amplitude (envelope) of the transmitted signal and part is encoded in the phase of the transmitted signal.
To avoid distorting communicated information, the power amplifier (PA) and various other components of a radio transmitter have to be linear, which is to say for example that the functional relationship between the output power of the PA and the input power of the PA is a straight line for all possible power levels. In addition, the phase shift of the input signal through the PA has to be constant for all possible power levels.
Departures from amplitude linearity and phase constancy introduce distortion into the PA's output signal, such as spectral broadening that can disturb nearby communication channels. Amplitude/phase distortion (vector distortion) in the transmitter can also increase the bit error rate (BER) of the communication system, e.g., degrading the audio quality of a voice call or reducing the speed of an internet connection.
In general, the likelihood of proper transmitter performance can be increased by including in the transmitter a transmitter observation receiver (TOR) that samples the output signal of the PA and generates a compensation signal that is fed back to the modulator, PA, and/or other transmitter components to correct the PA's output signal. In effect, the compensation signal pre-distorts the transmitter input signal such that the PA's output signal is apparently undistorted. Since transmitter distortion typically arises mainly in the PA, a signal acquired after the PA is fed back and compared with the transmitter input signal as part of the pre-distortion process.
FIG. 1 is a block diagram of an arrangement 100 that is an example of a pre-distortion-compensated transmitter having an antenna 102, a coupler 104, a power amplifier 106, a modulator 108, and a TOR 110. The PA 106 and modulator 108 can be considered the “transmit path” of the arrangement 100. It will be understood that the modulator 108 typically includes oscillators and other components not shown and that the modulator 108 generally represents the base-band processing and up-conversion processing applied to the input signal. As seen in FIG. 1, the TOR 110 samples the transmitted signal generated by the transmit path through the operation of the coupler 104 and provides a compensation signal to the modulator 108.
Currently available pre-distortion-compensated transmitters are generally designed to operate over a small range of transmitted frequencies, such as a communication band of a communication system. For example, the Long Term Evolution (LTE) communication system currently being standardized by the Third Generation Partnership Project (3GPP) has a communication Band 1that covers 2110-2170 megahertz (MHz). Both the forward transmit path and the feedback compensation path in the transmitter are effectively tuned to the same range of frequencies, and cannot be deployed to support other frequency ranges, e.g., other communication bands. The typical transmitter operation is constrained to a single (narrow) frequency range of interest as a result of spectral linearity limitations of its various tuned circuits (e.g., narrow-band filters) and tunable circuits (e.g., voltage-controlled local oscillators). For example, amplitude and phase variation over frequency makes linearization (pre-distortion) difficult over a broad range of frequencies, and an oscillator may be able to tune over only a few hundred MHz.
FIG. 2 is a block diagram that depicts a known way to use a single TOR in a single-frequency, multi-transmitter arrangement. The multiple transmitters generate respective signals having the same carrier frequency, e.g., any carrier in a communication band, of a communication system. In the arrangement 200 depicted in
FIG. 2, an antenna 202-1 receives output signals of a Tx 1 PA 206-1 and an antenna 202-2 receives output signals of a Tx 2 PA 206-2 that have a respective Tx 1 modulator 208-1 and a Tx 2 modulator 208-2. Couplers 204-1, 204-2 provide portions of the output signals of the PAs to a single TOR 210 through operation of a switch 212. The TOR 210 samples the output signal connected to it by the switch 212 without needing tuning and provides a compensation signal to the respective modulator. In this way, the single TOR 210 is essentially time-shared sequentially between the PAs 206-1, 206-2, each PA producing a signal in the same frequency range. It is believed that such an arrangement was available from Nortel in its CDMA tri-sector radio.
The frequency limitations of TORs and pre-distortion-compensated transmitters are becoming more serious problems as the number and range of available communication bands around the world increases. Currently available pre-distortion-compensated transmitters require redesign, modification and re-banding to operate in new communication bands, and this increases the cost of designing and supporting communication systems.